In the list of administrators that we can see, there are multiple names, but one user is shown with a SID Security Identifier instead of their domain\username. That particular username could not be resolved in the domain controller.

In our case, this was determined to be a user which was deleted from windows, or a user whose profile had been recreated.

To Fix it:

Highlight the orphaned SID in the list of users on the "Analysis Server Properties" dialogue on the "Security" pane, and click the Remove button.